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According to UX spec, UnifiedSystemTray supports collapsed state.

This CL
* Adds SetExpanded to FeaturePodButton. If false, it hides labels.
* Adds SetExpanded to FeaturePodsContainerView. If false, it uses layout
  that places all the buttons horizontally.
  Buttons exceed the limit will be hidden in the collapsed state.
  To support this behavior, FeaturePodButton::SetExpandedByContainer is
  also added.
* Adds SetExpanded to UnifiedSystemTrayView and puts all sliders to
  |sliders_container_|, so that it can be easily hidden when collapsed.
* Adds FeaturePodsContainerViewTest unit tests.

Screencast: http://go/scrcast/NTk3Mzg1NzQwMzkyODU3NnxhZTNmMDI2ZS1kNA

UnifiedSystemTray design doc: go/cros-qs-restyling

TEST=FeaturePodsContainerViewTest
BUG=819942

Change-Id: Ic7b7bcf56f1405b9eb0babab8f041f5a83d361f2
Reviewed-on: https://chromium-review.googlesource.com/964031Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544988}